In general, in order to suppress an adjacent channel interference with an FM receiver, the pass band width of the intermediate frequency amplifier stage is decreased; that is, the characteristic curve of selectivity is made sharp so that the radio interference of a station close in broadcast frequency to the aimed (or selected) station is sufficiently eliminated. However, the system is disadvantageous in that the demodulated signal is distorted, and the separation is lowered.
In order to overcome the above-described difficulty in a conventional adjacent channel interference suppressing system for an FM receiver, as shown in FIG. 1, a wide-band filter 3 and a narrow-band filter 4 are provided between a front end 1 and an intermediate frequency amplifier and detector unit 2. Depending on the conditions of adjacent channel interference, the wide-band filter or the narrow-band filter 4 is connected between the front end 1 and the intermediate frequency amplifier and detector unit 2 by means of gang switches 5a and 5b which are operated simultaneously. When there is no adjacent channel interference; i.e., the radio wave condition is good, the wide-band filter 3 is selected by means of the gang switches 5a and 5b so that the demodulated signal has little distortion; that is, the reproduced sound is high in quality. When there is an adjacent channel interference; i.e., the radio wave condition is poor, the narrow-band filter 4 is selected by means of the gang switches 5a and 5b, so that the radio interference of the station which is close in frequency to the aimed (or selected) station is eliminated so that the broadcast signal of the aimed or selected station can be positively received.
In the above-described conventional system either the wide-band filter 3 or the narrow-band filter 4 is merely inserted into the circuit by operating the switches 5a and 5b. Therefore, with the conventional device, the band width cannot be changed smoothly with the different conditions of adjacent channel interference, and the switching of the filters may give a sense of inconsistency to the operator (or listener).
If the circuit is so designed that, in switching the two filters, the intermediate frequency signals of the filters are continuous in wave form, then the sense of inconsistency may be lessened. However, the system is still disadvantageous in that, since the two filters are different in delay time from each other, it is difficult to make the wave forms continuous, and therefore the switching of the filters cannot be conducted without giving a sense of inconsistency to the operator.